Hi Trevor. Your idea looks very good. I leave the OIL in the usual place; it's freezing (5) definitely POLAR and the reference to (heating) oil would fit in nicely.
Don't know which "Enigma" this is. The puzzle is in a magazine called The Tablet. Probably bought in. I'll email them and enquire. Since it's a "bottle of wine" prize xword they may not wish to answer.
Thanks again T, I've now no doubt you're absolutely correct. John